1. Field of the Invention
The present invention relates to an optical transformer, capable of optically measuring voltages of components which are charged to high voltages, e.g., components of a power sub-station.
2. Description of the Related Art
Hitherto, an optical transformer has been proposed in which a voltage applied to a high-voltage conductor is potential-divided and thus divided voltage is applied to an electro-optical element. FIG. 1 shows a conventional optical transformer, used in a tubular gas-insulated electrical device. A high-voltage conductor 2 as a line conductor is received in a tubular metallic vessel 1, and an intermediate electrode 3 is disposed between the metallic vessel 1 and the high-voltage conductor 2. The potential of the intermediate electrode 3 is transmitted to the exterior of the metallic vessel 1 through a sealed terminal 4 which is electrically insulated from the metallic vessel 1. A main capacitor 5 is formed by a drifting capacitance between the high-voltage capacitor 2 and the intermediate electrode 3. A capacitance 6 is a drifting capacitance formed between the intermediate electrode 3 and the metallic vessel 1. In order to optimumly adjust the divided potential of the intermediate electrode 3, a capacitor 7 is connected between the intermediate electrode 3 and the earth, in parallel with the drifting electrostatic capacitance 6. The metallic vessel 1 is charged with an insulating gas 8, in order to enhance the insulation between the high-voltage conductor 2 and the metallic vessel 1.
The voltage between the intermediate electrode 3 and the ground is applied to an optical voltage sensor 10 through lead lines 9. A signal processing circuit 12, which is connected to the photoelectric sensor 10 by means of an optical fiber 11, transmits and receives light to and from the photoelectric sensor 10. The light from the optical voltage sensor 10 has an intensity modulated in proportion to a voltage applied to the optical voltage sensor 10. The signal processing circuit 12 converts the intensity-modulated light to a required electrtical amount.
FIG. 2 shows a circuit which is equivalent to that shown in FIG. 1. A capacitor 13 is a potential-dividing capacitor composed of the drifting electrostatic capacitance 6 and the capacitor 7.
When a voltage is applied to a high-voltage conductor 2, the voltage or potential of the intermediate electrode 3 is determined by the potential division performed by the main capacitor 5 and the potential-dividing capacitor 13, and this voltage is applied to the optical voltage sensor 10. Consequently, a voltage proportional to the voltage of the high-voltage conductor 2 is applied to the optical voltage sensor 10. The signal processing circuit 12 transmits light to the optical voltage sensor 10 through the optical fiber 11, and receives the light modulated by the optical voltage sensor 10 again through the optical fiber 11. In this state, the optical voltage sensor 10 modulates the light in proportion to the voltage applied thereto. It is therefore possible to measure the voltage of the high-voltage conductor 2, by processing the light signal by means of the signal processing circuit 12.
According to this arrangement, it is impossible to obtain an exact output when a trouble exists in the optical voltage sensor 10, optical fiber 11 or the signal processing circuit 12. In such a case, a device connected to the output of the signal processing circuit 12, e.g., a relay (not shown), may operate erroneously. In order to avoid such an inconvenience, it has been proposed to employ a duplicate construction composed of optical voltage sensors 10a, 10b, optical fibers 11a, 11b and signal processing circuits 12a, 12b. In case of a trouble in one of the optical fibers 11a and 11b and the signal processing circuit 12a and 12b, a difference occurs between the outputs of the two signal processing circuits. It is possible to avoid erroneous operation of the relay or the like device by using, for example, a voltage balance relay or the like, by making use of the difference in the outputs. This arrangement is shown as a fault detection circuit 17 in FIG. 3. This fault detection circuit 17 monitors electrical signals representative of the voltages output from the signal processing circuits 12a, 12b and applied across the optical voltage sensors 10a, 10b and detects occurrence of a fault upon sensing any difference between these electrical signals. A similar fault detection function may be applied also to detect fault in the optical voltage sensor 10a or 10b.
In the conventional optical transformer having the construction shown in FIG. 3, the optical voltage sensors 10a and 10b are connected to the same potential dividing capacitor 13 in parallel with each other. Therefore, in the event of a short-circuit failure in one of the optical voltage sensors 10a and 10b, the voltage across the potential dividing capacitor 13 is reduced to zero, so that both signal processing circuits 12a, 12b produce outputs corresponding to zero voltage. It is therefore impossible to prevent erroneous operation of the relay on the basis of the difference between the outputs of both signal processing circuits 12a, 12b.